Traveling wave tube with notched interdigital slow wave structure



Feb. 9, 1965 R. A. PAANANEN 3,1i69,2fi@

TRAVELING WAVE TUBE WITH NOTCHED INTERDIGITAL. SLOW WAVE STRUCTURE Filed May 5, 1958 A! Tram/E v States ate This invention concerns a traveling wave tube and more particularly a traveling wave tube having an interdigital slow wave propagating structure which is constructed so as to reduce electron beam interception by the tips of the elements comprising the slow wave structure.

Traveling wave tubes of both the forward wave and backward wave type are known which include a slow Wave energy propagating structure or delay line, a continuous electrode or sole spaced from and disposed substantially parallel to said structure, and an electron gun mounted adjacent one end of said structure for producing an electron beam which, under the influence of an electric field existing between said structure and said sole and a magnetic field transverse to the electric field and to the beam, traverses the interaction space bounded by the slow wave structure and the sole. Such a tube is discussed and described in a copending application of Edward C. Dench, Serial No. 733,228, filed April 30, 1958, now US. Letters Patent No. 3,084,277, issued April 2, 1963. Because of the interaction between the electron beam and high frequency fields of wave energy propagating along the slow wave structure, energy may be transferred from the electron beam to the high frequency field. A high frequency signal introduced into one end of said structure may be amplified in this manner; alternatively, the tube may be made to oscillate as a result of this energy exchange between the electron beam and the high frequency field.

One form of delay structure which has proven effective in traveling wave tubes is the interdigital delay structure which comprises a first assembly including a first set of elongated elements or fingers each having one extremity attached to a common base arranged substantially parallel to the longitudinal axis of the delay structure and a second assembly including a second set of elongated fingers each having one extremity attached to a common base which usually is arranged more or less parallel to the common base of the first assembly. The fingers of the first and second sets are intermeshed. The other extremity of each finger of a given assembly approaches but does not contact the common base of the other assembly.

In prior traveling wave tubes using interdigital delay structures, the electron beam, as the result of interaction with the high frequency field existing along the delay structure, quite often ispartially or completely intercepted by the fingers of the delay structure. Furthermore, in the eventof strong oscillations, for example, the

greater portion of the electron beam may impinge upon only a small proportion of the total number of fingers of the delay structure. The electron beam concentration on these few fingers normally is such that substantially the same amount of beam current is intercepted by a given length of finger near the tip of a finger as by the same length near the root (junction of the finger with the common base) of the same finger. Since the heat generated in the fingers owing to electron impingement obviously is much less readily dissipated in the tip than strike the root of the adjoining finger.

enteci Felo 5, 19%5 at the root of each finger, the tip of the finger may be overheated, even to the point of destruction. In some instances, the current density, for various reasons, may not be uniform along the length of each finger. Some or" the reasons for the lateral nonuniformity of the electron beam may be the existence of a non-uniform magnetic field, a non-uniformly emitting cathode or slight mechanical irregularities in the tube. In such instances, either the even or the odd-numbered fingers would have greater heat dissipation near the tips, while the oppositelynumbered fingers would have less heat dissipation near the roots. This unbalanced condition is particularly detrimental in that the set of fingers interceptive of electrons at the tips are much more liable to melt than fingers which intercept electrons near the root.

In accordance with this invention, the delay structure is designed so that, whether or not the current density distribution is uniform, most of the electron beam current impinges upon the portion of the finger nearest to the root; this portion, upon which electron impingement mostly occurs, may extend, for example, about one-half or two-thirds of the length of each finger from the corresponding root. This is accomplished by removing the tips of the fingers back away from the path followed by the major portion of the electron beam, that is, by increasing the distance between the tips or free extremities of the fingers and the sole electrode. The fingers may be notched on the side facing the electron beam. The amount of retraction perpendicular to the interaction space may be relatively small. The retraction of the finger tips may be either stepped or a gradual tapering of the finger so that the spacing between the finger and the sole electrode increases as the free end of said finge is approached. Also in accordance with the invention, the tips of the fingers may be bent mechanically away from the sole electrode either during or before tube assembly. With the above arrangements, most or" the current normally intercepted by a finger tip will, instead, By minimizing the interception of electrons by the finger tips and permitting the electrons to strike the finger roots, a more effective dissipation of heat can be effected.

Other objects and features of this invention will be understood more clearly and fully from the following detailed description of the invention with reference to the accompanying drawing wherein:

FIG. 1 is a cross-sectional view, partly in elevation, of a traveling wave tube according to the invention;

FIG. 2 is a fragmentary section view of the traveling wave tube of FIG. 1 taken along the line 2-2;

FIG. 3 is a detail view of a portion of the slow wave structure of the tube of FIGS. 1 and 2;

FIG. 4 is a fragmentary view of a portion of a linear version of the tube of FIG. 1;

FIG. 5 is a detail view showing a portion of the slow wave structure and sole of the tube of FIG. 4; and

FIGS. 6 and 7 are views showing modifications of a delay line such as may be used in the tubes of FIG. 1 or FIG. 4.

Referring to the drawing, a traveling wave oscillator tubeltl is shown which comprises a slow wave energy propagating structure or delay line 12, a cylindrical electrode I4, otherwise referred to as a sole, concentric with the delay line 12 and normally maintained negative with respect thereto, a lead-in assembly 15, an electron gun assembly 26 including at least a cathode 21 and heater 22, a magnetic field-producing means 25 and an output tube 1% after evacuation.

coupling means 17. The circular delay line 12 includes several interdigital fingers or elements 31 and 32 extending from respective oppositely disposed annular common base members 33 and 34. Members 33 and 34 are secured, as by screws 35, to the shoulder portion of a cylindrical electrically-conductive ring 36, said ring forming a part of the evacuated envelope of tube 10. The remainder of the delay line 12 includes a pair of oppositely located cover plates 33 and 39 hermetically sealed to ring 36.

The sole 14 consists essentially of a cylindrical block of electrically conductive material, having a peripheral surface 26 and preferably bounded laterally by outwardly tlared edge extensions 27. T he purpose of the extensions 27 is to confine the electron beam within the interaction space St between surface 26 of sole 14 and the interdigital line 12. One end of a hollow supporting member 54 is inserted within the inner periphery of a centrally located aperture in sole 14 and is afiixed to the sole. Supporting member 54, in addition to providing mechanical support for sole 14, forms a portion of lead-in assembly 15 and allows for passage of external circuit-connecting leads, in a manner to be discussed later. Sole 14 contains a slot 23 to accommodate the electron gun assembly 26. Since the invention does not involve details of the electron gun, the latter is shown in simplified form in FIGS. 1 and 2. The construction and manner of mounting of the electron gun 7.9 may be as indicated in the copending application of Roy A. Paananen, Serial No. 717,897, filed February 27, 1958, now US. Letters Patent No. 2,914,700, issued November 24, 1959. Electron gun 20 includes a cathode 21, a heater 22, a grid electrode 23, which may be used for control of beam current (as for amplitude modulation) and an accelerating electrode 24, which also may be used for control of beam current. The cathode 21 may have a rectangular body coated with an clectron-emissive material and provided with a circular bore in which a heater wire 22 is inserted. One end of the heater may be connected to the inner wall of the cathode. Electrical energy from appropriate sources is supplied to the cathode 21, eater 22, grid electrode 23 and accelerating electrode 24 by way of respective lead-in wires 61, 62, 63, and 64, which extend from the tube envelope through the lead-in assembly 15. in FIG. 1, the cathode only of the electron gun is shown, for the sake of simplicity and clarity.

Lead-in assembly 15 includes an electrically conductive sleeve 66 atlixed to the inner periphery of cover plate 38, as indicated in l. A section of cylindrical glass tubing 67 interconnects sleeve es and a second electricallyconductive sleeve 63. The other end of sleeve 53 is provided with a glass seal 69 for sealing the traveling wave One end of sole-supporting member 54 contains an outwardly flared portion, not shown, which is connected to the inner surface of tube 68. The leads 61, e2, 63, and64 are mounted in electricallyinsulated relation with supporting member by one or more glass beads '11.

The coaxial output coupling means 17 is sealed in an opening of wall 3-5 of delay line 12 and is impedancematched to the delay line. The inner conductor 73 of the output coupling means 17 is connected to a finger of delay line 12 at or near the end of the delay line adjacent electron gun 2%.

Traveling wave tube it may be provided with a collector electrode 75, shown in 2, for intercepting electrons after one traversal of the arcuate interaction space 511. This collector electrode may be in the form of a projection from wall to delay structure 12. In some instances, however, the collector electrode may be omitted and the electron stream made reentrant.

The no essary electric field between the slow-wave structure 12 and sole 14 may be obtained by means of a unidirectional voltage applied therebetween; such a voltage may be supplied by a battery The sole 14 may be biased negatively relative to the cathode 21 by means of a source 81 of voltage connected between cathode lead at and sole-supporting member 54 by way of tube 68. The cathode 21. may in some instances, however, be at the same potential as sole 14; in this case, source 81 would be omitted. Similarly, the clay line 12 may be maintained at a potential positive relative to both sole 14 and catntode 21 by means of the source 33 of unidirectional voltage connected between the cathode and sleeve 66, the latter being connected, in turn, to delay line 12. The accelerating electrode 24 may be maintained at a potential positive relative to cathode 21 by means of a source 85 of unidirectional voltage connected between leads 61 and 64. The control grid lead 63 may be connected by way of terminal 83 to an appropriate energy source for controlling the magnitude of the electron beam current in the traveling wave oscillator 10. Heater voltage may be supplied by means of a source 82 interconnecting cathode and heater leads 61 and 62. a

A uniform magnetic field transverse to the direction of propagation of the electron beam is provided by a permanent magnet or electromagnet having cylindrical pole pieces 91 and $2 radially positioned on or adjacent the anode cover plates 38 and 39, respectively. Pole piece 91 is apertured to receive lead-in assembly 15, while pole piece 92 is similarly apertured to maintain symmetry of the magnetic field. The flux lines should be concentrated in the interaction space 5tibetween sole 14 and delay line 12. By proper adjustment or": the magnitude and polarity of the magnetic and electric fields so established, the electron beam may be made to follow a more or less circular path along the interaction space 50 under the combined influence of these transversely disposed fields.

Although the device of FIGS. 1 and 2 is indicated as an oscillator having but a single external energy coupling means, and, as shown in FIG. 2, would operate as a backward wave oscillator, it should be understood that this invention is not limited to circular tubes which oscillate or to circular tubes which oscillate in the backward mode. For example, by the addition of a coupling structure similar to structure 1'7 coupled to the other end of the delay line 12, it is possible to adapt the circular tube of FIGS. 1 and 2 as an amplifier. The fingers 31 and 32 of the interdigital delay line 12 are provided with retracted portions 31' and 32', respectively, adjacent the free extermities of the fingers, as most clearly shown in FIG. 3. By notching the fingers adjacent the tips, much of the electron beam which' may impinge upon the delay line 12 will travel past the notched (retracted) portions 31' of tingers 31 and strike the root portion of adjacent fingers 32; similarly, the electron beam will tend to travel past the retracted portions 32' oi fingers 32 and strike the root portions of adjacent fingers 31. Heat may readily be dissipated from the root portions by way of the relatively massive portions 33 or 34 and the cylinder 36.

This invention may also be incorporated in a nonreentrant tube, or a linear tube, of the type indicated in FIGS. 4 and 5, in which elements corresponding to those of FIGS. 1 to 3 are indicated by like reference numerals. Such a tube is described completely in a copending application of J. M. Osepchuk and R. A. Paananen, Serial No. 388,473, filed October'27, 1953, now Patent No. 2,905,- 859, issued September 22, 1959. The tube 10 of FIGS. 4 and 5 include a metallic envelope portion made up of end plates andside plates 62 and 63 respectively, only one of each being visible in FIG. 4, as Well as top and bottom plates 64 and 65. Tube 10 includes an interdigital delay line 12 comprising a pair of opposed parallel common base members, one of which, namely member 33, is visible in FIG. 4; the delay line also comprises a plurality of interdigital fingers 31 and 32 which extend from respective base members 33 and 34. A cathode structure 20' is positioned beyond one end of delay line 12 and includes a cathode support member 95 which, for example, may be an annular member extending through an aperture in plate 64 and insulatedly sealed by means of seal 96 to a member 97. The latter, in turn, is sealed to the aforesaid aperture in plate 64 of the tube envelope. The cathode 20' may, for example, be an indirectly heated cathode attached to the inner end of support member 95. The cathode may enclose a heater wire, not shown, one end of which may be connected to the cathode and the other end of which may be connected to a lead-in Wire 98 which extends through support member 95 and is spaced from and insulatedly sealed to the end of support member 95 by an insulating seal 99. The cathode 20 is coated with an electron-emissive material 100. A sole electrode 14 extends parallel to the delay line 12 and is supported by means of lead-in supports 101 extending out through the plate 64 of the tube envelope and insulated therefrom. Attached to the delay line finger 31 adjacent the cathode 20 is a signal output coupling structure 17 from which high frequency energy may be extracted from the delay line 12 and supplied to a load. The device as described in FIG. 4 is adapted to operate as a backward wave oscillator. An additional coupling structure, however, may be coupled to the other end of the delay line 12 of the tube of FIG. 4, in which case one of the coupling structures may be receptive of an input signal to be amplified and the other structure used as an output coupling structure. Furthermore, a single output structure could be used at the end of the tube opposite that shown in FIG. 4. The desired arrangement of the output coupling structure or structures, as the case may be, depends upon whether operation as a backward wave amplifier, backward wave oscillator, forward wave amplifier or forward wave oscillator is desired.

The fingers 31 and 32 of the delay line 12 of FIGS. 4 and 5 are notched at portions 31' and 32, respectively, as in the case of the tube of FIGS. 1 to 3.

A modification of the notched-finger delay line previously shown is illustrated in FIG. 6 wherein the retracted portions 31 and 32' are tapered, that is, the spacing between the tips of the fingers and the sole 14 is progressively increased as the end of the finger is approached.

Another modification of the delay line is shown in FIG. 7, wherein the tips of the fingers are bent downwardly rather than having material removed from the tips, as indicated, for example, in the delay lines of FIGS. 5 and 6. The portions 31' and 32' of the fingers of FIG. 7 are all substantially the same length as those of the previously shown embodiments; likewise, the amount of retraction of the fingers of FIG. 7 at the free extremities may be of the same order of magnitude as that of the previously shown fingers.

It should be noted that the modifications shown in FIGS. 6 and 7 are not necessarily limited to linear tubes of the type shown in FIG. 4, but obviously may be applied equally well to a circular tube of the type shown in FIGS. 1 to 3.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. A traveling wave electron discharge device comprising an interdigital slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated thereby and means for directing electrons in a beam along said region in energy exchanging relation with said fields of Wave energy for collection by one end of said structure, said one end of said structure comprising a pair of elongated base members spaced from one another and a plurality of periodically spaced elements extending from respective oppositely disposed base member parallel to said region and having one extremity free, each of said elements being notched at said free extremity facing said electron beam and confined to a region adjacent the free extremity of said corresponding element.

2. A traveling wave electron discharge device comprising an interdigital slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated thereby, an electrode spaced from and substantially coextensive with said slow wave structure, and means for directing electrons in a beam along said region in energy exchanging relation with said fields of wave energy for collection by one end of said structure, said one end of said structure comprising a pair of elongated base members spaced from one another and a plurality of periodically spaced elements extending from respective oppositely disposed base members parallel to said region and having one extremity free, each of said elements being notched at said free extremity facing said beam and confined to a region adjacent the free extremity thereof, said notch serving to remove said free extremity farther from said beam from said electrode than the remainder of said corresponding element.

3. A traveling wave electron discharge device comprising an interdigital slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated therebetween, an electrode spaced from and substantially coextensive with said slow wave structure, and forming thereby an interaction space, and means for directing electrons in a beam along said interaction space in energy exchanging relation with said fields of wave energy for collection by one end of said structure, said one end of said structure comprising a pair of elongated base members spaced from one another and a plurality of periodically spaced elements extending from respective oppositely disposed base members parallel to said region and having one extremity free, each of said elements having a retracted portion having at least one notch facing said beam and confined to a region adjacent the free extremity thereof.

4. A traveling wave electron discharge device comprising an interdigital slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated thereby, an electrode spaced from and substantially coextensive with said slow wave structure, and means for directing electrons in a beam along said region in energy exchanging relation with said fields of wave energy for collection by one end of said structure, said one end of said structure comprising a pair of elongated base members spaced from one another and a plurality of periodically spaced elements extending from respective oppositely disposed base members parallel to said region and having one extremity free, each of said elements having a step at said one extremity facing said beam and confined to a region adjacent said free extremity, said notch serving to position said free extremity further from said electrode than the remainder of said corresponding element as the free extremity thereof is approached.

5. A traveling wave electron discharge device comprising an interdigital slow Wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated thereby, an electrode spaced from and substantially coextensive with said slow wave structure, and means for directing electrons in a beam along said region in energy exchanging relation with said fields of wave energy for collection by one end of said structure, said one end of said structure comprising a pair of elongated base members spaced from one another and a plurality of periodically spaced elements extending from respective oppositely disposed base members parallel to said region and having one extremity free, each of said elements being notched at said free extremity facing said electron beam and confined to a region adjacent said free extremity, said notch being 7 positioned further from said electrode than the remainder 2,890,372 of said corresponding element.

References Cited in the file of this patent 699,890 UNITED STATES PATENTS 5 1,102,594 2,730,678 Dohler et a1 Jan. 10, 1956 1,104,861 2,770,780 Warnecke et a1. Nov. 13, 1956 7731709 8 Bench June 9, 1959 FOREIGN PATENTS Great Britain Nov. 18, 1953 France May 11, 1955 France Nov. 24, 1955 Great Britain May 1, 1957 

4. A TRAVELING WAVE ELECTRON DISCHARGE DEVICE COMPRISING AN INTERDIGITAL SLOW WAVE ENERGY PROPAGING STRUCTURE PRODUCING IN THE REGION ADJACENT THERETO FIELDS OF ELECTROMAGNETIC WAVE ENERGY BEING PROPAGATED THEREBY, AN ELECTRODE SPACED FROM SAID SUBSTANTIALLY COEXTENSIVE WITH SAID SLOW WAVE STRUCTURE, AND MEANS FOR DIRECTING ELECTRONS IN A BEAM ALONG SAID REGION IN ENERGY EXCHANGING RELATION WITH SAID FIELDS OF WAVE ENERGY FOR COLLECTION BY ONE END OF SAID STRUCTURE, SAID ONE END OF SAID STRUCTURE COMPRISING A PAIR OF ELONGATED BASE MEMBERS SPACED FROM ONE ANOTHER AND A PLURALITY OF PERIODICALLY SPACED ELEMENTS EXTENDING FROM RESPECTIVE OPPOSITELY DISPOSED BASE MEMBERS PARALLEL TO SAID REGION AND HAVING ONE EXTREMITY FREE, EACH OF SAID ELEMENTS HAVING A STEP AT SAID ONE EXTREMITY FACING SAID BEAM AND CONFINED TO A REGION ADJACENT SAID FREE EXTREMITY, SAID NOTCH SERVING TO POSITION SAID FREE EXTREMITY FURTHER FROM SAID ELECTRODE THAN THE REMAINDER OF SAID CORRESPONDING ELEMENT AS THE FREE EXTREMITY THEREOF IS APPROACHED. 